ND Filter

Many small form factor cameras such as the IXUS/SD series only have one real aperture setting, which is commonly f/2.8 at wide-angle (ie: when the camera is not zoomed in at all).

The lens in all point and shoot cameras (and some D/SLR zoom lenses) have a variable maximum aperture value (lowest f number), which means if a camera manufacturer lists the camera with the following zoom specs 6.2 - 18.6mm and aperture f/2.8 - 4.9.
In this case, the maximum aperture will be f/2.8 when the camera is at its' widest focal length (6.2mm) and increases as you zoom in until you reach the telephoto setting (of 18.6mm) to an f-stop maximum value of f/4.9. That is, when zoomed in at 18.6mm your camera is at f/4.9 and can not go any lower (ie: to f/2.8).

Which is why you will get different aperture values when zooming in, despite these cameras having a single fixed aperture.

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How the ND filter in these cameras work

When your camera detects a really bright scene, it'll swing the ND filter in. This in effect makes the scene look darker and will effect the exposure (but not depth of field like a real aperture would).

With the ND filter in, the camera can select a slower shutter speed without over-exposing the entire scene.
Also, with the ND filter swung in, the camera will capture an image with an aperture of f/8 - This is a fake aperture value caused by the ND filter swinging in.

With CHDK, you can keep the ND filter out, and just select a faster shutter speed to compensate.
In CHDK, you can perform this operation by going to:

Extra Photo Operations -> ND filter state

Just select one of the options from there: Off/In/Out

'Off' means the camera is in control of the ND filter state.

'In' means the ND filter is swung in.

'Out' means the ND filter is not in.

Notes:

setting the ND position in a script takes effect just before the actual shot is take and after the exposure is set

Not all cameras have an ND filter; all cameras of the Ixus series have a ND Filter instead of a real aperture, also some other ones, e.g. the A450, A460, A530, A550, A560, A1000, TX1.

Some higher end cameras have both an ND filter and a real aperture, e.g the cameras of the G series.